The present invention concerns a sliding journal bearing.
In sliding journal bearings with hydrodynamic lubrication, the problem exists in maintaining the formation of the lubricant wedge between the bearing shell and the shaft with certainty over the complete rotational speed and load range and to attain good shaft guidance with the smallest possible movement of the shaft axis from the required position. In order to obtain these properties, bearing shells have been proposed with the most varied cross-sections normal to the axis. The simplest cross-section known for lightly loaded sliding journal bearings is the circular one with a clearance relative to the shaft such that, after attaining a certain peripheral speed of the shaft journal, a lubricant wedge capable of carrying load is formed by hydrodynamic effects and separates the sliding surfaces of the shaft and the bearing shell from one another. Support is then supplied in the region of liquid friction or "floating friction", after the shaft has previously had to pass through the mixed friction region below the peripheral speed mentioned above. This simplest design of the bearing running surface has the disadvantage that the shaft axis is displaced in operation relative to the rest position, the displacement becoming greater as the lubricant wedge becomes more strongly formed. The location of the shaft journal on the bearing shell is then "single line", i.e. along a generatrix of the shaft journal surface. In this respect, the obtention of a "two line" support of the shaft in the bearing sliding surface, or "lemon clearance" wherein the bearing shell or bearing half shells are widened in a wedge shape in the region above and below the dividing plane, is a solution to the shaft displacement problem caused by a "single line" support. Two diametrically opposed lubricant wedges are formed during operation and provide double line location. A variant of this concept having the same effect is the "displaced lemon clearance" of Klemencic. This method also has double line location. In the case of high accuracy bearings, such as are necessary for the main spindles of lathes, milling machines and other highly loaded machine tools, there are special designs with three and four line location and, consequently, three or four lubricant wedges.
For the lubrication of the two bearing types last mentioned, reliance is not generally placed on the build-up of a hydrodynamically occurring lubricant wedge and lubrication film. Rather, the lubrication is hydrostatically ensured by a continuous supply of oil under pressure. Such pressure circulation lubrication also guarantees, in association with an oil cooler, oil filter, etc., the removal of the bearing frictional heat, this being a precondition for reliable, continuous operation.
As far as the two bearing types mentioned with lemon clearance are concerned, these are generally designed in two parts with a horizontal dividing plane because of the method of manufacture. Relative to the first mentioned single shell construction with a circular bore, this implies more complicated and expensive manufacture. These inhibiting considerations must be balanced against the advantage of the double line location and better shaft axis centering already mentioned.
Still more expensive, of course, are the bearing types mentioned having three and four line location for higher requirements with respect to load carrying capacity, reliable lubrication and centering of the shaft axis.